Three areas of concentration will contribute to the over-all goal of elucidating the mechanisms governing intestinal calcium transport and its regulation. 1) Investigate the relative roles of cellular and paracellular routes of calcium transport using the paracellular solute probe technique under short-circuit conditions. 2) Establishing the site of action of agents known to modulate calcium absorption using the above technique (e.g. vitamin D, ATPase inhibitors, hormones). 3) Detail the influence of anomalous paracellular secretory effects on magnesium, strontium, cadmium, copper, zinc, and manganese using the paracellular probe methodology. 4) Continue the investigation of calcium flux across isolated microvillus and basolateral membranes distinguishing membrane bound from transported calcium.